A typical vehicle hybrid hydraulic drive system uses a reversible pump/motor to absorb power from and add power to or assist a conventional vehicle drive system. The system absorbs power by pumping hydraulic fluid from a low pressure reservoir into a hydraulic energy storage system. This hydraulic energy storage system typically includes one or more nitrogen-charged hydraulic accumulators. Hybrid hydraulic drive systems typically add power to conventional vehicle drive systems by utilizing the hydraulic energy stored in the hydraulic accumulators to drive the reversible pump/motor as a motor.